Baclofen: Interaction with GABAB Receptor Leading to CNS Depression Poster Team: Elizabeth Kelling,Nicole Kurszewski, Melissa Sachse, Madeline Schober, Robert Wolf Jmol Team: Andrew Ackerman, David Antoine, Jonathan Foust, Jillian Godlewski, Andrew Saether Faculty Advisors: Daniel Sem, Ph. D. and Ernest Stremski, MD, MBA Institution: Concordia University Wisconsin School of Pharmacy, Mequon, WI 53097

Abstract Molecular Story Future Research

Trauma, such as spinal shock, is a major cause of muscle A current challenge with baclofen administration is the drug’s Amine2,3 spasticity leading to rigid muscles and intense pain. GABAB ability to cause CNS depression. Baclofen acts on GABAB receptor agonists, such as baclofen, are a common treatment for receptors both in the brain and spinal cord. This undesirable In vivo, the amine group on baclofen has a spasticity because they are effective in reducing muscle tone and, activation of GABA receptors in the brain can cause clinically positive charge that is essential for the hydrogen B thus, muscle spasticity. The treatment for long term pain significant sedative effects.1 Currently, baclofen is being binding interactions with the GABA receptor. This management following such traumas often includes opioid B administered intrathecally to produce a localized effect in the group interacts with the amino acids Histidine-170, analgesic medications as well. Providers prescribing both GABA spinal cord. One possibility for further improvement in the B Glutamate-349, and Tryptophan-278 via hydrogen agonists with opioid analgesic should be cautious as concomitant mechanism of action of baclofen is to give it higher affinity for bonds. Two of the protons on the amine interact use of these drugs can cause increased central nervous system the GABA receptors in the spinal cord. The enhancement of with the double bonded nitrogen in the imidazole B (CNS) side effects such as respiratory depression, loss of receptor specificity could lead to the possibility of other, more ring of Histidine-170 and the nucleophilic oxygen consciousness, and hypotension.1 accommodating dosage forms. Having baclofen preferentially in Glutamate-349. The nitrogen in the amine bind in the spinal cord over the brain would decrease the CNS interacts with the phenyl group of Tryptophan-278 depressant effects that are most concerning regarding patient via van der Waals hydrophobic interactions in the Introduction safety. active sight. (Figure 2) Figure 2. Hydrogen bonding interactions between Baclofen’s amine A 55 year old male was admitted to the ED after being ejected group and Histidine-170 and Glutamine-349. Figure also displays the from his vehicle. He suffered from multiple scapular and thoracic amine undergoing van der Waals interactions with Tryptophan-278. Rendered from 4MS4.pdb spine fractures resulting in spinal shock. The patient experienced severe muscle spasticity and pain associated with his trauma. As a result, the patient was started on the maximum dose of 80 mg per day of baclofen by mouth for spasticity, as well as fentanyl and 2,3 oxycodone to manage his pain. The pharmacist closely monitored Carboxylic Acid Fig. 5 Downstream signaling of the GABAB receptor the patient for signs and symptoms of CNS depression and Obtained from: Golan Figure 12.54 adjusted doses of therapies accordingly. In vivo, the carboxylic acid group on baclofen is deprotonated, resulting in a negative charge that allows it to interact with different amino Summary acids compared to the amine group. The main Baclofen2 amino acids that the carboxylic acid group Baclofen is a medication used to treat muscle spasticity, which • GABA analogue interacts with are Serine-130, Serine-153, and works by acting as an agonist to the GABAB receptor. The • First line treatment for muscle spasticity Tyrosine-250. Both oxygen atoms, with two lone GABAB receptor is a metabotropic G-coupled protein receptor • Acts as a selective agonist targeting GABA receptors B pairs on the carboxylic acid group, form multiple that functions to hyperpolarize the cell and decrease action • Concomitant use of baclofen with opioid analgesics can enhance hydrogen bonds with Serine-153 and Serine- potentials, causing relaxation of the muscle cells. Baclofen binds the effects of CNS side effects leading to respiratory depression or 130. Tyrosine-250 only forms one hydrogen the GABAB receptor via hydrogen bonds and van der Waals sedation bond to the carboxylic acid group. (Figure 3) interactions. The various components of the chemical structure • Binds six amino acids in the active site of the GABA receptor Fig. 3 B of baclofen binds to six different amino acids on the GABAB via van der Waals and hydrogen bonds receptor.

Since GABAB receptors are found in both the spinal cord and Figure 3. Hydrogen bonding between the carboxylic acid of Baclofen and the amino acid serine-130,Serine-153 and Tyrosine-250. Rendered from brain, patients taking baclofen should be cautious when taking 4MS4.pdb other medications that cause sedation. Opioid analgesic medication can lead to increased CNS effects, such as respiratory depression, sedation, and hypotension and should be used with caution in conjunction. In our case, the patient was Beta-Chlorphenyl2,3 closely monitored for any signs or symptoms of CNS depression. The beta-chlorphenyl group in baclofen partakes in the van der Waals hydrophobic interactions with Tyrosine-250 and Tryptophan-278. This ring interacts with these two amino acids to create References Figure 1. Baclofen Structure molecular ring stacking. The indole ring of Obtained from: alcalc.oxfordjournals.org 1. Hudgeson, P.; Weightman, D. Baclofen in the Treatment of Spasticity. British Medical Journal. Tryptophan-278 flips around to accommodate the 1971;4: 15-17. 2. Geng Y, Bush M, et al. Structural mechanism of ligand activation in human GABA(B) receptor. 2 beta-chorphenyl ring substituent of baclofen, GABA Receptor Nature. 2013 Dec 12;504(7479):254-9. doi: 10.1038/nature12725. B allowing the formation of the aromatic ring • Inhibitory receptor that stops neurotransmitter release 3. Wieronska JM, Stachowicz K, Nowak G, Pilc A. The loss of Glutamate – GABA Harmony in stacking interactions. (Figure 4) Anxiety Disorders. In: Kalinin V, ed. Anxiety Disorders. Kraków, Poland; 2011. • Metabotropic transmembrane G-coupled protein receptor, made http://www.intechopen.com/books/anxiety-disorders/the-loss-of-glutamate-gaba-harmony-in- anxiety-disorders. Fig. 4 up of 2 subunits, GBR1 and GBR2 4. Golan DE, Tashjian AH, Armstrong EJ, Armstrong AW, eds. Principles of Pharmacology: The • When bound with GABA, potassium channels open leading to an Pathophysiologic Basis of Drug Therapy. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins Kluwer; 2012. efflux of potassium, which causes hyperpolarization of the cell Figure 4. Pi stacking interaction between the Beta-Chlorphenyl group of • Stimulation of inhibitory neurons decreases action potentials and baclofen and Tyrosine-250 and Tryptophan-278. Rendered from 4MS4.pdb causes sedation.

The CREST Program is funded by NSF-DUE grants #1022793 and #1323414.